Method for forming a lateral super-junction MOSFET device and termination  structure

ABSTRACT

A method for forming a lateral superjunction MOSFET device includes forming a semiconductor body including a lateral superjunction structure and a first column connected to the lateral superjunction structure. The MOSFET device includes the first column to receive current from the channel when the MOSFET is turned on and to distribute the channel current to the lateral superjunction structure functioning as the drain drift region. In some embodiment, the MOSFET device includes a second column disposed in close proximity to the first column. The second column disposed near the first column is used to pinch off the first column when the MOSFET device is to be turned off and to block the high voltage being sustained by the MOSFET device at the drain terminal from reaching the gate structure. In some embodiments, the MOSFET device further includes termination structures for the drain, source and body contact doped region fingers.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/971,624, entitled METHOD FOR FORMING A LATERAL SUPER-JUNCTION MOSFETDEVICE AND TERMINATION STRUCTURE, filed May 4, 2018, now U.S. Pat. No.10,243,072, issued Mar. 26, 2019, which is a continuation of U.S. patentapplication Ser. No. 15/632,204, entitled LATERAL SUPER-JUNCTION MOSFETDEVICE AND TERMINATION STRUCTURE, filed Jun. 23, 2017, now U.S. Pat. No.9,991,380, issued Jun. 5, 2018, which is a continuation of U.S. patentapplication Ser. No. 15/051,438, entitled LATERAL SUPER-JUNCTION MOSFETDEVICE AND TERMINATION STRUCTURE, filed Feb. 23, 2016, now U.S. Pat. No.9,722,073, issued Aug. 1, 2017, which is a continuation of U.S. patentapplication Ser. No. 14/747,925, entitled LATERAL SUPER-JUNCTION MOSFETDEVICE AND TERMINATION STRUCTURE, filed Jun. 23, 2015, now U.S. Pat. No.9,312,381, issued Apr. 12, 2016, all of which are incorporated herein byreference for all purposes.

BACKGROUND OF THE INVENTION

Semiconductor devices incorporating superjunction structure to achieveimproved electrical characteristics are known. For example, metal oxidesemiconductor field effect transistor (MOSFET) devices can beincorporated with vertical or horizontal superjunction structure tooptimize the on-resistance and the breakdown voltage characteristics ofthe transistor. As an example, Fujihira describes configurations of thelateral and vertical superjunction devices in the paper entitled “Theoryof Semiconductor Superjunction Devices” (Japan Journal of AppliedPhysics Vol. 36, October 1997 PP 6254-6262). U.S. Pat. No. 6,097,063also describes a vertical semiconductor device having a drift region inwhich a drift current flows if the drift region is in the ON mode andwhich is depleted if the drift region is in the OFF mode. The driftregion is formed as a structure having a plurality of first conductivetype divided drift regions and a plurality of second conductive typecompartment regions in which each of the compartment regions ispositioned among the adjacent drift regions in parallel to make p-njunctions, respectively.

Challenges remain in the design and manufacturing of superjunctionsemiconductor devices. These challenges include the difficulties informing the superjunction structure, difficulties in improvingmanufacturability, and high product costs when epitaxial processes areused, among others. Furthermore, termination of the superjunctionstructure is important to ensure robust device operation.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1 is a perspective view of a lateral superjunction MOSFET device inembodiments of the present invention.

FIG. 2 is a cross-sectional view of the lateral superjunction MOSFETdevice of FIG. 1 along a line A-A′ in embodiments of the presentinvention.

FIG. 3 is a cross-sectional view of the lateral superjunction MOSFETdevice of FIG. 1 along a line B-B′ in embodiments of the presentinvention.

FIG. 4 is a perspective view of a lateral superjunction MOSFET device inalternate embodiments of the present invention.

FIG. 5 is a cross-sectional view of the lateral superjunction MOSFETdevice of FIG. 4 along a line C-C′ in embodiments of the presentinvention.

FIG. 6 is a top view of the high voltage MOSFET device formed usinglateral superjunction MOSFET cell in embodiments of the presentinvention.

FIG. 7 is a top view of the lateral superjunction MOSFET device of FIG.6 incorporating a termination pillar structure in embodiments of thepresent invention.

FIGS. 8 and 9 illustrate alternate embodiments of the termination pillarstructure as applied to an N+ doped region, such as a drain finger or abody finger, in a high voltage MOSFET device.

FIG. 10 is a cross-sectional view of the termination pillar structure inMOSFET device 80 of FIG. 7 along a line D-D′ in embodiments of thepresent invention.

FIG. 11 is a top view of is a top view of the lateral superjunctionMOSFET device of FIG. 7 incorporating a termination pillar structurewith RESURF surface implant in embodiments of the present invention.

FIGS. 12A to 12J are cross-sectional view showing the processing stepsto form a lateral superjunction structure using the ion implantationfabrication method in embodiments of the present invention.

FIGS. 13A and 13B illustrate the doping profiles in the lateralsuperjunction structure fabrication method of the present inventionbefore and after annealing in embodiments of the present invention.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; and/or a composition of matter. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

According to embodiments of the present invention, a lateralsuperjunction MOSFET device includes a MOS gate structure, an N-typecolumn connected to the lateral superjunction structure and a P-typecolumn disposed in close proximity to the N-type column. The MOS gatestructure can be a low voltage gate structure, such as a planar gateconfigured to withstand only a portion of the voltage sustained by theMOSFET device. The lateral superjunction MOSFET device includes theN-type column to receive current from the channel when the MOSFET isturned on and to distribute the channel current to the N-type layers inthe lateral superjunction structure. The channel current flows throughthe N-type superjunction layers and is collected by the drain terminalat the far end of the lateral superjunction structure. The P-type columndisposed near the N-type column is used to pinch off the N-type columnwhen the MOSFET device is to be turned off and to block the high voltagebeing sustained by the MOSFET device at the drain terminal from reachingthe MOS gate. The P-type column can be connected to the source/bodyvoltage of the MOSFET device.

In the present description, a superjunction structure refers to asemiconductor device structure including a thin semiconductor region ofa first conductivity type functioning as a conduction channel of thesemiconductor device and is bordered or sandwiched by thin semiconductorregions of a second, opposite conductivity type to form a balanced spacecharge region for enhancing the breakdown voltage characteristic of thesemiconductor device. In some applications, the superjunction structureincludes multiple thin semiconductor regions of alternating conductivitytypes formed laterally or vertically. That is, a superjunction structureincludes alternating thin N-type semiconductor regions and thin P-typesemiconductor regions that may be formed laterally or vertically. Themultiple thin semiconductor regions of alternating N and P conductivitytypes are sometimes referred to herein as superjunction layers. In thepresent description, a lateral superjunction structure includessuperjunction layers that extend substantially laterally in thesemiconductor chip, that is, substantially in parallel with the majorsurfaces of the semiconductor chip. Accordingly, current flows in thelateral superjunction structure in a lateral direction through thesuperjunction layers or in parallel to the major surface of thesemiconductor chip. On the other hand, a vertical superjunctionstructure includes superjunction layers that extend substantiallyvertically in the semiconductor chip, that is, substantiallyperpendicular with the major surfaces of the semiconductor chip.Accordingly, current flows in the vertical superjunction structure in avertical direction through the superjunction layers or perpendicular tothe semiconductor chip.

A salient feature of the lateral superjunction MOSFET device is that thesurface gate or planar gate does not extend the entire depth of thelateral superjunction structure. Conventional superjunction MOSFET orJFET devices are formed using a trench gate that extends the entiredepth of the superjunction structure. These conventional superjunctionMOSFET or JFET devices thus suffer from high gate capacitance whichlimits the switching speed of the transistor device. In embodiments ofthe present invention, the lateral superjunction MOSFET device is formedusing a surface planar gate to realize a small gate capacitance toensure faster transistor switching speed.

The operation of the lateral superjunction MOSFET device of the presentinvention is as follows. When the MOSFET is turned on, a channel isformed in the body region under the low-voltage MOS gate and channelcurrent flows from the source through the channel. The channel currentfeeds into the N-type column which distributes the current into theN-type superjunction layers connected thereto as the drain driftcurrent. The drain drift current flows through the N-type superjunctionlayers to be collected by a drain terminal formed at the far end of thesuperjunction structure. When the MOSFET is turned on, the N-type columnis therefore electrically connected to the drain with the drain beingbiased to a low drain voltage. When the MOSFET is turned off, the drainterminal is driven up to a large drain voltage (e.g. 600V). However, theP-column, connected to the source or body or the ground potential,pinches off the N-type column so that the N-type column floats and willnot be driven up to the large drain bias voltage. In this manner, theP-column isolates the MOS gate from the high voltage sustained at thedrain terminal while the transistor is turned off and a low voltage gatestructure can be used. A low voltage MOS gate structure is desirable forlower gate capacitance and faster switching time.

According to other embodiments of the present invention, a lateralsuperjunction MOSFET device incorporates an edge termination structurefor the lateral superjunction structure using N or P type terminationcolumns or pillars. In other embodiments, the edge termination structurefor the lateral superjunction structure further incorporates single ormulti-step field plates to the N or P type termination pillars. In yetother embodiments, the edge termination structure for the lateralsuperjunction structure further includes a RESURF (Reduced SurfaceField) shallow surface implant to reduce the surface field strength toachieve a breakdown voltage for the MOSFET device.

According to other embodiments of the present invention, a fabricationmethod to form a lateral superjunction structure in a semiconductordevice uses N and P type ion implantations into a base epitaxial layer.In some embodiments, the base epitaxial layer is an intrinsic epitaxiallayer or a lightly doped epitaxial layer. In some embodiments, themethod performs simultaneous N and P type ion implantations into thebase epitaxial layer. The epitaxial and implantation processes arerepeated successively to form multiple implanted base epitaxial layerson a substrate. After the desired number of implanted base epitaxiallayers is formed, the entire semiconductor structure is subjected tohigh temperature annealing. The difference in diffusion rates for the Ptype and the N type dopants is used to form a lateral superjunctionstructure including alternate N and P type thin semiconductor regions.In particular, the alternating N and P type thin superjunction layersare formed by the ion implantation process and subsequent annealing. Thefabrication method of the present invention ensures good charge controlin the lateral superjunction structure.

In particular, conventional fabrication methods for forming a lateralsuperjunction structure typically use successive epitaxial layers ofalternating conductivity types. However, epitaxial processes typicallyhave large variations in thickness and in doping concentration. As aresult, a superjunction structure formed using thin epitaxial layerstypically has poor charge control. That is, the desired layer thicknessand dopant concentration for the thin semiconductor layers cannot beobtained. The fabrication method of the present invention uses dopantimplantation into intrinsic or lightly doped epitaxial layers andannealing to form the superjunction structure. Implant processes givemuch better control over doping concentration than epitaxial processes.When intrinsic or lightly doped epitaxial layer is used as the baselayer, the epitaxial doping and/or thickness variations have no effecton the charge balance of the superjunction structure. Instead, thecharge balance of the superjunction structure is controlled by implantprocesses forming the N-type and P-type layers, which implant processescan be very tightly controlled. For example, implant processes canachieve doping and thickness variations of 2% or less typically. Suchtight control over doping and thickness variations is not achievable byusing epitaxial processes.

Lateral Super-Junction MOSFET Device

In embodiments of the present invention, a lateral superjunction MOSFETdevice uses a low voltage MOS gate structure. The lateral superjunctionMOSFET device includes an N-type column connected to the lateralsuperjunction structure and a P-type column disposed in close proximityto the N-type column. The N-type column and the P-type column operate inconjunction to enable the MOSFET device to sustain a high voltage whileisolating the low voltage MOS gate structure from the high voltage beingsustained.

FIG. 1 is a perspective view of a lateral superjunction MOSFET device inembodiments of the present invention. FIG. 2 is a cross-sectional viewof the lateral superjunction MOSFET device of FIG. 1 along a line A-A′in embodiments of the present invention. FIG. 3 is a cross-sectionalview of the lateral superjunction MOSFET device of FIG. 1 along a lineB-B′ in embodiments of the present invention. Referring to FIGS. 1-3, alateral superjunction MOSFET device 10 is formed on a heavily dopedP-type substrate 11 (“P+ substrate”). A lightly doped P-type epitaxiallayer 12 is formed on the P+substrate 11. The P+ substrate 11 and theP-type epitaxial layer 12 form a P-type semiconductor base layer 13 onwhich the MOSFET device is formed. In the present description, theP-type semiconductor base layer 13 will be referred to as a “P− baselayer.” In the present embodiment, an N-type buried layer (NBL) 24 isformed on the P− base layer 13. The use of the lightly doped P-typeepitaxial layer 12 and the N-type buried layer 24 has the effect ofimproving the breakdown sustainability of the MOSFET device, as will beexplained in more detail below.

The lateral superjunction MOSFET device 10 includes a semiconductor body25 with a lateral superjunction structure formed therein. Morespecifically, the semiconductor body 25 includes thin semiconductorregions of alternating N and P type conductivities. In particular, thesemiconductor body 25 includes P-type thin semiconductor regions 25 aand N-type thin semiconductor regions 25 b that are formed alternatelyand extend substantially laterally in the semiconductor body. That is,the P-type thin semiconductor regions 25 a and N-type thin semiconductorregions 25 b, also referred herein as the superjunction layers, areformed substantially in parallel with the major surfaces of thesemiconductor body 25. In the present description, the semiconductorbody 25 is also referred to as the lateral superjunction structure 25.

To form the MOS transistor structure, the lateral superjunction MOSFETdevice 10 includes a low voltage gate structure formed on or in thesemiconductor body 25 at a near end of the lateral superjunctionstructure. In the present embodiment, a planar gate structure is used asthe low voltage gate structure. As shown in FIG. 1, a planar conductivegate 14 is formed on the top surface of the semiconductor body 25 and isinsulated from the semiconductor body 25 by a thin gate dielectric layer15. In some embodiments, the planar conductive gate 14 is a polysilicongate and the gate dielectric layer 15 is a gate oxide layer. The lateralsuperjunction MOSFET device 10 further includes an N+ source region 16formed in a P-type body region 19 (“P-body”) and formed self-aligned toa first end of the conductive gate 14. As thus formed, the N+ sourceregion 16 extends under the first end of the planar conductive gate 14,overlapping the conductive gate by a small amount. A P+ body contactregion 18 is formed adjacent the N+ source region 16 and in the P-bodyregion 19 for providing an ohmic contact to the P-body region of theMOSFET device. The gate 14 and the source 16 are formed at one end ofthe lateral superjunction structure 25. Meanwhile, an N+ drain region26, formed as an N+ drain column, is formed at the distant end of thelateral superjunction structure 25 with the superjunction layersfunctioning as the drain drift region of the MOSFET device.

An insulating dielectric layer 30 is formed over the top surface of thesemiconductor body 25 and openings in the dielectric layer 30 are madeto form contacts to the source, body and drain of the MOSFET device 10.In the present embodiment, a contact opening is made to the N+ source 16and the P+ body contact region 18 and a metal electrode 32 is formed inthe contact opening as the source/body electrode. Another contactopening is made to the N+ drain region 26 and a metal electrode 34 isformed in the contact opening as the drain electrode. The heavily dopedP+ substrate 11 forms a second source electrode of the MOSFET device,forming a bottom source electrode. The P+ substrate 11 provides a lowinductance path to the ground terminal, which can improve the switchingwaveforms of the transistor significantly. The bottom source electrodealso provides a path for avalanche current to flow directly to theground terminal via the vertical diode formed by the N+ drain column 26and the N-type buried layer (NBL) 24, to the P-type epitaxial layer 12and the P+ substrate 11. In the present embodiment, to reduce the drainresistance, a doped polysilicon filled trench 28 is formed in the N+drain column 26. The doped polysilicon filled trench 28 is optional andmay be omitted in other embodiments of the present invention.

In lateral superjunction MOSFET device 10, the superjunction layers 25a, 25 b function as the drain drift region of the MOSFET device with thethin semiconductor regions of one conductivity type functioning as thedrain current paths to carry the drain drift current in the transistorOn-state and the thin semiconductor regions of the other conductivitytype functioning as a charge-balanced partition region to pinch off ordeplete the drain current paths in the transistor Off-state. For theN-type MOSFET device 10, the N-type thin semiconductor regions 25 b formthe drain current paths to carry the drain current from the sourceregion 16 to the drain region 26 while the P-type thin semiconductorregions 25 a form the charge-balanced partition region which aredepleted in the transistor off-state to deplete and pinch off the N-typethin semiconductor regions 25 b.

In embodiments of the present invention, the lateral superjunctionMOSFET device 10 includes an N-type column 20 disposed under the gate 14and being spaced apart from the source region 16 with the separationbetween the source region 16 and the N-type column 20 being the channelregion of MOSFET device. The N-type column 20 is electrically unbiased.The N-type column 20 extends in a vertical direction through the lateralsuperjunction structure 25. In some embodiments, the N-type column 20 isa heavily doped N+ region and is electrically connected to the draincolumn via the N-type superjunction layers at low drain bias conditions,when the MOSFET is turned on. However, the N-type column 20 iselectrically floating at high drain bias conditions, when the MOSFET isturned off. In particular, at higher drain biases of 50V and above, thesuperjunction layers 25 a and 25 b will deplete out, thereby eliminatingthe connection between the N-type column 20 and the N+ drain column 26.In this manner, the N-type column 20 is isolated from the high drainvoltage.

In embodiments of the present invention, the lateral superjunctionMOSFET device 10 includes a P-type column 22 formed in spaced apart butin close proximity to the N-type column 20. The P-type column 22 is alsoreferred to as the P-type blocking column. In the present embodiment,the P-type column 22 is formed in vertical alignment to the P+ bodycontact region 18 and is electrically connected to the P-body region 19of the MOSFET device. Therefore, the P-type column 22 is biased to thesame electrical potential as the body region of the MOSFET device. TheP-type column 22 is not a continuous doped region through the width ofthe semiconductor body 25. Rather, the P-type column 22 can be formed asa single column or pillar occupying a portion of the semiconductor body25 in the z-direction along the width of the superjunction structure, asshown in FIG. 1. Alternately, the P-type column 22 can be formed toinclude separate P-type columns or pillars, such as P-type columns 22 aand 22 b, disposed in the z-direction along the width of thesuperjunction structure, as shown in FIG. 1. Accordingly, while theP-type column 22 interrupts the drain current paths formed in the N-typethin semiconductor regions 25 b in some locations (FIG. 2), the N-typethin semiconductor regions 25 b remain contiguous and connected in otherlocations (FIG. 3) along the width of the superjunction structure.

As thus configured, the lateral superjunction MOSFET device 10 of thepresent invention is able to achieve a high breakdown voltage whileoptimizing the on-resistance of the transistor. The operation of thelateral superjunction MOSFET device 10 is as follows. The N+ source andP-body regions of the MOSFET device are connected to a ground potentialor to a negative power supply potential. When the MOSFET device 10 isturned on by the application of a positive voltage to the gate 14relative to the source region 16 that is greater than the thresholdvoltage of the transistor, a channel is formed in the P-body region 19under the gate 14 between the source region 16 and the N-type column 20.The channel connects the N+source region to the N-type column 20. When apositive voltage is applied to the drain electrode 34, current flowsfrom the source region 16 to the drain region 26. In particular, achannel current flows from the source region 16 through the channelunder the gate 14 and feeds into the N-type column 20. The N-type column20 distributes the current into the N-type superjunction layers 25 bconnected thereto as the drain drift current. The drain drift currentflows through the N-type superjunction layers 25 b to be collected bythe drain region 26 at the far end of the superjunction structure 25. Inthis manner, the lateral superjunction MOSFET device 10 is able toachieve a low on-resistance.

When the lateral superjunction MOSFET device 10 is turned off by theapplication of a voltage to the gate 14 less than the threshold voltageof the transistor device, the P-type column 22, biased to the bodypotential, is depleted and the depletion region extends to pinch off theN-type column 20. The superjunction layers 25 a and 25 b are alsodepleted completely to isolate the N-type column 20 from the N+ draincolumn 26. With the N-type column 20 thus isolated by the P-type column22 and the superjunction layers, the N-type column 20 will not getdriven up to the high drain voltage (e.g. 600V). In some embodiment, theN-type column is clamped at a voltage of 10V or below while the drainterminal sustains a large drain voltage (e.g. 600V) with the transistorbeing turned off.

In this manner, the P-type column 22 protects the MOS gate 14 from thehigh voltage sustained at the drain region 26 and a low voltage gatestructure can be used in the MOSFET device 10. In particular, a lowvoltage MOS gate structure is desirable for lower gate capacitance andfaster switching time. In some embodiments, the gate 14 of the MOSFETdevice 10 may be configured to sustain a low voltage of 20V while thedrain may be configured to sustain a high voltage of 600V.

The lateral superjunction MOSFET device 10 thus formed is capable ofsustaining a high breakdown voltage through the use of the lateralsuperjunction structure. Furthermore, in embodiments of the presentinvention, the lateral superjunction MOSFET device 10 includes theN-type buried layer 24 formed under the drain region 26. The N-typeburied layer 24 further improves the vertical breakdown voltage of theMOSFET device.

FIG. 4 is a perspective view of a lateral superjunction MOSFET device inalternate embodiments of the present invention. FIG. 5 is across-sectional view of the lateral superjunction MOSFET device of FIG.4 along a line C-C′ in embodiments of the present invention. Referringto FIGS. 4-5, a lateral superjunction MOSFET device 50 is constructed inthe same manner as the lateral superjunction MOSFET device 10 of FIG. 1except with the placement of the MOS gate structure and the N-typecolumn. In particular, lateral superjunction MOSFET device 50 includes alateral superjunction structure 65 formed on a lightly doped P-typeepitaxial layer 52 formed on a P+ substrate 51. The lateralsuperjunction structure 65 functions as the drain drift region for theMOSFET device. An N+ drain region 66 is formed at a far end of thelateral superjunction structure 65. An N-type buried layer (NBL) 64 isformed under the N+ drain region 66 to enhance the breakdown voltage ofthe MOSFET device.

An insulating dielectric layer 70 is formed over the top surface of thesemiconductor body 65 and openings in the dielectric layer 70 are madeto form contacts to the source, body and drain of the MOSFET device 50.In FIG. 5, a contact opening is made to the N+source 56 and the P+bodycontact region 58 and a metal electrode 72 is formed in the contactopening as the source/body electrode.

In FIG. 1, the lateral superjunction MOSFET device 10 has the gatestructure and the N-type column configured so that when the channel ofthe transistor is turned on, the channel current runs in a directionparallel to drain current paths formed by the superjunction layers. Inthe embodiment shown in FIG. 4, the lateral superjunction MOSFET device50 has the gate structure 54 and the N-type column 60 configured so thatwhen the channel of the transistor is turned on, the channel currentruns in a direction perpendicular to the drain current paths formed bythe superjunction layers. More specifically, the current flowing fromthe source region 56, through the channel under the gate 54 is collectedby the N-type column 60 which distributes the current to the N-typesuperjunction layers 65 b. The drain drift current from the N-typecolumn 60 and flowing through the drain drift region formed by thelateral superjunction structure 65 travels in a perpendicular directionto the channel current. The drain drift current is then collected by theN+ drain electrode 66 at the other end of the lateral superjunctionstructure 65.

In the embodiment shown in FIGS. 4 and 5, the gate 54 of the MOSFETdevice 50 and the N+ column 60 are flanked by P-type columns 62 and 62a. P-type columns 62 and 62 a are formed in spaced apart but in closeproximity to the N-type column 50. In the present embodiment, the P-typecolumn 62 is formed in contact with the P+ body contact region 58 and iselectrically connected to the P-body region 59 of the MOSFET device.Therefore, the P-type column 62 is biased to the same electricalpotential as the body region of the MOSFET device 50. In the presentembodiment, N+ column 60 is flanked by the P-type column on both sides.In other embodiments, only one P-type column, such as P-type column 62may be used. The P-type columns 62, 62 a operates to pinch off the N+column 60 when the MOSFET device 50 is to be turned off to isolate thegate 54 of the transistor from the high voltage that may be sustained atthe drain terminal of the transistor.

FIGS. 1-5 illustrate two different configurations of the low voltage MOSgate structure, the N-type column and the P-type column. One of ordinaryskilled in the art would appreciate that the exact configurations of thelow voltage MOS gate structure, the N-type column and the P-type columnin the MOSFET device described above are illustrative only and notintended to be limiting. Other configurations of the low voltage MOSgate structure, the N-type column and the P-type column may be used inthe lateral superjunction MOSFET device of the present invention as longas the N-type column is used to distribute the current from the channelof the transistor to the lateral superjunction structure when thetransistor is turned on and the P-type column functions to deplete andpinch off the N-type column as well as isolating the low voltage MOSgate when the transistor is turned off.

Termination Structures

In the lateral superjunction MOSFET device described above, edgetermination techniques are employed to manage the high electrical fieldthat may develop at the end or the termination of the drain and/orsource regions of the MOSFET device.

FIG. 6 is a top view of the high voltage MOSFET device formed usinglateral superjunction MOSFET cell in embodiments of the presentinvention. In embodiments of the present invention, the lateralsuperjunction MOSFET device described above in FIGS. 1-5 may be used asa basic MOSFET cell where the MOSFET cell is duplicated to form an arrayof MOSFET devices where the MOSFET cells are connected in parallel toform a high voltage MOSFET device. In some embodiments, the basic MOSFETcell may be repeated to form MOSFET cells connected in parallel torealize a high voltage MOSFET integrated circuit. FIG. 6 illustrates aportion of a high voltage MOSFET device 80 where the MOSFET device 10 ofFIG. 1 is used as the basic MOSFET cell that is duplicated and mirroredto form an array of parallelly connected MOSFET devices. As thusconfigured, the N+ drain regions 26 of the MOSFET cells extend from thedrain pad 86 into the active cell area and P+ body contact region 18extends from the source/body pad 82 into the active cell area. P-typecolumns 22 may be formed in alignment with the P+ body contact region18. The polysilicon gate 14 is formed above the body region and the N+column (not shown) and is connected to a gate pad 84. The source regionis formed adjacent the gate 14 and is not shown in FIG. 6 forsimplicity.

As thus configured, the N+ drain regions 26 form long fingers (“drainfingers”) in the MOSFET device 80. The ends of the drain fingers formtermination regions 90 which may experience concentrated electricalfield due to the geometry of the drain fingers. Similarly, the P+ bodycontact regions 18 form long fingers (“body contact fingers”) in theMOSFET device 80. The ends of the body region fingers form terminationregions 92 which may experience concentrated electric field due to thegeometry of the body region fingers.

In embodiments of the present invention, a termination structure for alateral superjunction MOSFET device uses N-type or P-type terminationcolumns or pillars at the termination regions of respective N+ or P+doped regions. In particular, N-type or P-type termination pillars orcolumns are vertical doped regions formed in the semiconductor body thatextends into the semiconductor body to a depth similar to the depth ofthe N+ or P+ doped regions to be protected. In some embodiments, thetermination pillars or columns are electrically floating, that is, notelectrically connected to a specific potential. In other embodiments,the termination pillars may be biased to a given voltage from thesurrounding doped region the pillars are in physical contact with. Forexample, in some embodiments, the P-type termination pillars may beweakly connected to the source potential and the N-type terminationpillars may be weakly connected to the drain potential at zero drainbias. However, once the drain bias is sufficient to pinch-off thesuperjunction layers, the N-type termination pillars will float, andshape the termination electric field by reaching intermediate electricpotential. FIG. 7 is a top view of the lateral superjunction MOSFETdevice of FIG. 6 incorporating a termination pillar structure inembodiments of the present invention. In FIG. 7, the gate layer 14 isomitted to simplify the drawing and to better illustrate the terminationstructure of the present invention. Referring to FIG. 7, the terminationstructure includes N-type termination pillars 102 formed in thetermination regions of the N+ drain fingers 26. Meanwhile, P-typetermination pillars 104 are formed in the termination regions of P+ bodycontact fingers 18. The termination pillars 102 and 104 improve thebreakdown characteristics of the MOSFET integrated circuit 100.

In embodiments of the present invention, the number and positioning ofthe termination pillars are selected to optimize the breakdowncharacteristics of the MOSFET device. In the embodiment shown in FIG. 7,a pair of linearly aligned termination pillars is used in thetermination region of each doped region finger. The number andarrangement of the termination pillar structure in FIG. 7 areillustrative only and not intended to be limiting. In other embodiments,one or more pillar pillars may be used. Furthermore, a given pattern orarrangement of the pillar pillars may be used to optimize the breakdowncharacteristics of the MOSFET device. FIGS. 8 and 9 illustrate alternateembodiments of the termination pillar structure as applied to an N+doped region, such as a drain finger or a body finger, in a high voltageMOSFET device. Referring to FIG. 8, a termination structure 110 for anN+ drain finger 26 includes a linear series of four N-type terminationpillars arranged in the termination region of the N+ drain finger 26.Referring to FIG. 9, a termination structure 120 for an N+ drain finger26 includes a two-dimensional arrangement of N-type termination pillarsin the termination region of the N+ drain finger 26. The exact numberand arrangement of the termination pillars in the termination region arenot critical to the practice of the present invention.

FIG. 10 is a cross-sectional view of the termination pillar structure inMOSFET device 80 of FIG. 7 along a line D-D′ in embodiments of thepresent invention. Referring to FIG. 10, P-type termination pillars 104are formed in the termination region of the body contact finger 18. TheP-type termination pillars 104 are formed in the semiconductor body 25with the lateral superjunction structure formed therein. Furthermore,the P-type termination pillars 104 extend through the semiconductor body25 to the P-type epitaxial layer 12. In some embodiments, the P-typetermination pillars 104 are formed in the same manner as the P-typecolumn 22.

Furthermore, in embodiments of the present invention, the terminationstructure for the lateral superjunction MOSFET device may furtherinclude a field plate formed on the top surface of the semiconductorbody 25 and surrounding the termination pillars 104 to shape the surfaceelectric field to further enhance the breakdown characteristics.Referring to FIG. 10, a field plate structure 160 is formed on the topsurface of the semiconductor body 25 and surrounding each P-typetermination pillar 104. In the present embodiment, a multi-step fieldplate is used. In other embodiments, single or multi-step field platemay be used to shape the surface electric field of the terminationpillars. In some embodiments, the field plate is formed using apolysilicon or metal layer. Furthermore, a multi-step field plate may beformed using polysilicon or metal with an overlying silicon oxide orsilicon nitride layer to form a multi-step field plate structure.

In embodiments of the present invention, the termination structure forthe lateral superjunction MOSFET device may further include a reducedsurface field (RESURF) surface implant. A RESURF surface implant is ashallow implant at the surface of the semiconductor body in thetermination region to shape the surface electric field and reduce thesurface field strength. FIG. 11 is a top view of is a top view of thelateral superjunction MOSFET device of FIG. 7 incorporating atermination pillar structure with RESURF surface implant in embodimentsof the present invention. Referring to FIG. 11, a P-type RESURF surfaceimplant region 190 is formed in the termination region of the bodycontact finger 18 while an N-type RESURF surface implant region 195 isformed in the termination region of drain finger 26. The RESURF surfaceimplant can be used in addition to the termination pillars to form amore rugged termination structure.

Furthermore, in embodiments of the present invention, the shape of theRESURF surface implant region 190, 195 can be adapted to optimize thefield shaping effect. In the present illustration, the P-type RESURFsurface implant region 190 is formed in a triangular shape while theN-type RESURF surface implant region 195 is formed in rectangular shape.Other shapes for the RESURF surface implant region can be used in otherembodiments of the present invention to shape the surface electric fieldbased on the electric field profile.

Method to Form Lateral Superjunction Structure

Conventional fabrication methods for forming a lateral superjunctionstructure typically use successive epitaxial layers of alternatingconductivity types to form the thin N and P type semiconductor layers.However, epitaxial processes are typically associated with largevariations in thickness and in doping concentration. For example, thethickness variation of an epitaxial process can have a variation of +/−5%. As a result, a superjunction structure formed using thin epitaxiallayers typically has poor charge control. That is, the desired layerthickness and dopant concentration for the thin semiconductor layerscannot be obtained. Therefore, lateral superjunction structures formedusing N and P type epitaxial layers cannot achieve the desired level ofcharge balancing required for optimal operation.

In embodiments of the present invention, a fabrication method to form alateral superjunction structure in a semiconductor device uses N and Ptype ion implantations into a base epitaxial layer. In some embodiments,the method performs simultaneous N and P type ion implantations into abase epitaxial layer. In some embodiments, the base epitaxial layer isan intrinsic epitaxial layer or a lightly doped epitaxial layer. Theepitaxial and implantation processes are repeated successively to formmultiple implanted base epitaxial layers on a substrate. After thedesired number of implanted base epitaxial layers are formed, the entiresemiconductor structure is subjected to high temperature anneal.Difference in the diffusion rates of the P type and N type dopants isused to separate out the P and N type dopants and to form lateralsuperjunction structure including alternate N and P type thinsemiconductor regions. In particular, the alternating N and P type thinsuperjunction layers are formed by the ion implantation process andsubsequent annealing. By the use of ion implantation to form the N and Ptype superjunction layers, the fabrication method of the presentinvention ensures good charge control in the lateral superjunctionstructure. More specifically, ion implantation processes give bettercontrol in doping concentration and doping profile and can thereforeensure tight doping concentration distribution in the lateralsuperjunction structure thus formed.

The fabrication method of the present invention uses ion implantationinto intrinsic or lightly doped epitaxial layers and annealing to formthe lateral superjunction structure. Ion implantation processes givemuch better control over doping concentration than epitaxial processes.When intrinsic or lightly doped epitaxial layer is used as the baselayer, the epitaxial doping and/or thickness variations have no effecton the charge balance of the lateral superjunction structure. Instead,the charge balance of the superjunction structure is controlled by ionimplantation processes forming the N-type and P-type layers, where theion implantation processes can be very tightly controlled. For example,implant processes can achieve doping and thickness variations of 2% orless typically. Such tight control over doping and thickness variationsis not achievable by using epitaxial processes alone to form the N and Ptype thin semiconductor layers.

FIGS. 12A to 12J are cross-sectional view showing the processing stepsto form a lateral superjunction structure using the ion implantationfabrication method in embodiments of the present invention. Referring toFIG. 12A, the fabrication process starts with a heavily doped P-typesemiconductor substrate 201. A lightly doped P-type epitaxial layer 202is grown on the heavily doped P+ substrate 201. The P+ substrate 201 andthe P-type epitaxial layer 202 form a semiconductor base layer 205 onwhich the lateral superjunction structure is to be formed. In otherembodiments, a lightly doped N-type (N−) silicon substrate can be used.

A blanket P-type ion implantation is performed to form a blanket P layer204 on the semiconductor base layer 205. The blanket P-type ionimplantation may be performed after a pad oxide layer is formed on thetop surface of the epitaxial layer 202. Then, a patterned N-typeimplantation process is carried out to form an N-buried layer 208 and apatterned P-type implantation process is carried out to form a P-buriedlayer 206.

With the semiconductor base layer 205 thus formed, the process forforming the lateral superjunction structure can begin. Referring to FIG.12B, a base epitaxial layer 210 is formed on the semiconductor baselayer 205. In some embodiments, the base epitaxial layer 210 is anintrinsic layer. In other embodiments, the base epitaxial layer is alightly doped layer, such as a lightly doped N− epitaxial layer or alightly doped P− epitaxial layer. Then, referring to FIG. 12C, N and Pion implantation is performed to implant N and P type dopants into thebase epitaxial layer 210. In some embodiments, the N and P type dopantsare implanted simultaneously and implanted at the same or substantiallythe same depth.

As a result of the implantation process, N type dopants 212 and P typedopants 214 are implanted in the base epitaxial layer 210. The implanteddopants have not yet been activated and the implanted base epitaxiallayer 210 contains the implanted dopants remaining at more or less theimplant site. A subsequent anneal process will be carried out toactivate the implanted dopants at which point the implanted dopants willspread to form the alternating N and P thin semiconductor regions, aswill be explained in more detail below.

The epitaxial and ion implantation process of FIGS. 12B and 12C isrepeated to form the desired number of lateral superjunction layers.Referring to FIG. 12D, a second base epitaxial layer 220 is formed onthe first base epitaxial layer 210. The second base epitaxial layer 220may be intrinsic or lightly N-type doped. Then, referring to FIG. 12E, Nand P ion implantation is performed to implant N and P type dopants intothe base epitaxial layer 220.

In the present embodiment, additional processing steps are alsoperformed to form a P-type column to be used for channel blocking and anN+ column to be used as the drain region. Referring to FIG. 12F, aP-type buried layer implant 226 is performed at a location verticallyaligned with the P-buried layer 206 previously formed in thesemiconductor base layer 205. Furthermore, an N-type buried layerimplant 228 is performed at a location vertically aligned with theN-buried layer 208 previously formed in the semiconductor base layer205. Then, the epitaxial and ion implantation process of FIGS. 12B and12C is repeated again to form another set of superjunction layers. Afterannealing, the P-buried layers will merge to form a P-type column. Andthe N-buried layers will merge to form an N-type column. Alternately,the P-type column can be formed by using a deep trench etch and P+polysilicon fill after the epitaxial growth process.

Referring to FIG. 12G, a third base epitaxial layer 240 is formed on thesecond base epitaxial layer 220. The third base epitaxial layer 240 maybe intrinsic or lightly N-type doped. Then, referring to FIG. 12H, N andP ion implantation is performed to implant N and P type dopants into thebase epitaxial layer 240. In the present example, it is assumed thatonly three layers of implanted base epitaxial layers are needed. Then,referring to FIG. 12I, a cap epitaxial layer 250 is formed on the thirdor last base epitaxial layer. The cap epitaxial layer 250 can beintrinsic or lightly N-doped.

In embodiments of the present invention, the first base epitaxial layer210 has a thickness of about 5 μm and the subsequent base epitaxiallayers 220, 240 have a thickness of about 2 μm. The cap epitaxial layer250 has a thickness of about 3 μm.

After the final epitaxial and implantation process, the entiresemiconductor structure of FIG. 12I is subjected to high temperatureanneal. For example, the semiconductor structure may be annealed at1150° C. for 200 minutes. The annealing process activates and spread outthe implanted dopants to form the desired alternating N and P type thinsemiconductor regions, as shown in FIG. 12J. After annealing, the N-typedopants spread to form the N-type superjunction layers 280B and theP-type dopants spread to form the P-type superjunction layers 280A. Alateral superjunction structure 280 is thus formed. Meanwhile, theP-type buried layer 226 and 206 are also annealed and spread to form acontiguous P-type column 270. The N-type buried layer 208 and 228 arealso annealed and spread to form a contiguous N-type column 260.

In embodiments of the present invention, the N and P type ionimplantation is performed using arsenic or antimony as the N-typedopants and boron as the P-type dopants. By using a heavier N-typedopant as compared to the P-type dopant, the N-type implanted dopants donot spread out far from the implanted site during annealing. Meanwhile,by using a lighter P-type dopant, the P-type implanted dopants spreadout from the implanted site farther during annealing to form a P-typelayer with uniform doping concentration. Furthermore, in embodiments ofthe present invention, the simultaneous N and P type ion implantation isperformed using a higher N-type implant dose than the P-type implantdose to ensure that the N-type doping concentration does not get washedout by the P-type dopants during the annealing process. In someembodiment, the N-type implant dose is three times the P-type implantdose. In this manner, alternating N and P-type thin semiconductorregions are formed by annealing the N and P implanted dopants formed inthe multiple base epitaxial layers.

It is instructive to note that the processing steps of the N-type andP-type buried layers are described to illustrate the formation of thevertical doped regions in the lateral superjunction structure and arenot critical to the practice of the present invention. Other methods forforming the vertical doped regions can be used in other embodiments ofthe present invention.

FIGS. 13A and 13B illustrate the doping profiles in the lateralsuperjunction structure fabrication method of the present inventionbefore and after annealing in embodiments of the present invention.Referring to FIG. 13A which illustrates the doping profiles after allimplantation processes but before the annealing operation, the baseepitaxial layer has an epitaxial doping level represented by curve 302.The simultaneous N and P type implants are performed to the same depthin each base epitaxial layer. The N-type dopant (curve 304) has a higherimplant dose than the P-type dopant (curve 306). Furthermore, the P-typedopant is lighter than the N-type dopant and therefore, the implantprofile of the P-type dopant is wider than the N-type dopant.

FIG. 13B illustrates the doping profiles after the annealing operation.The annealing process activates and spread out the implanted dopants.The N-type dopant does not spread as much as P-type and remains mostlyaround the implanted site. Meanwhile, after annealing, the P-typeimplanted dopants spread out to cover the base epitaxial layer to form asubstantially blanket P-type layer. The N-type implant has a high dopingconcentration and therefore the N-type doping concentration is notwashed out by the P-type dopants. In this manner, alternating N and Players are formed in the base epitaxial layers as shown by curve 310.

In the above described embodiments, an N-type MOSFET device isdescribed. It is understood that a P-type lateral superjunction MOSFETdevice can be constructed in a similar manner by reversing thepolarities of the doped regions.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A method for forming a lateral superjunctionMOSFET device, comprising: providing a semiconductor body comprising alateral superjunction structure including a plurality of alternatingN-type and P-type thin semiconductor regions formed substantially in sparallel with a major surface of the semiconductor body, the alternatingN-type and P-type thin semiconductor regions forming a drain driftregion of the MOSFET device; forming a body region of a firstconductivity type in a first surface of the semiconductor body andcomprising a plurality of body region fingers arranged laterally on thefirst surface of the semiconductor body; forming a conductive gate onthe semiconductor body at a near end of the lateral superjunctionstructure and being insulated from the semiconductor body by a gatedielectric layer, the conductive gate comprising a plurality ofconductive gate fingers arranged laterally on the semiconductor body,each conductive gate fingers being adjacent to and overlapping arespective body region finger; forming a source region of a secondconductivity type, opposite to the first conductivity type andcomprising a plurality of source region fingers, each source regionfinger being formed in a body region finger and self-aligned with afirst side of a conductive gate finger, each source region fingerextending under the first side of the respective conductive gate fingerto form a small overlap; forming a body contact region of the firstconductivity type and comprising a plurality of body contact regionfingers, each body contact region fingers being formed in a body regionfinger and adjacent to a source region finger formed therein; forming adrain region of the second conductivity type at a distant end of thelateral superjunction structure, the drain region extending through thelateral superjunction structure, the drain region comprising a pluralityof drain region fingers; forming a plurality of transistor cellsconnected in parallel, each transistor cell comprising a conductive gatefinger, a source region finger, a body contact region finger and a drainregion finger arranged laterally within each transistor cell, each ofthe drain region fingers and the body contact region fingers comprises aheavily doped region finger having a termination region at an end of theheavily doped region finger; and forming a termination structure in thetermination region of each heavily doped region finger, comprisingforming one or more lightly doped termination columns having the sameconductivity type as the heavily doped region finger and positioned nearthe end of the heavily doped region finger.
 2. The method of claim 1,wherein forming the source region, the body contact region and the drainregion comprises: forming the source region fingers and the body contactregion fingers on the first surface extending from a source-body pad ina first direction; and forming the drain region fingers on the firstsurface extending from a drain pad in a second direction, opposite thefirst direction.
 3. The method of claim 2, wherein forming the pluralityof transistor cells comprises: in each transistor cell, forming the bodycontact region finger interleaved between the source region finger andthe drain region finger.
 4. The method of claim 1, further comprising:forming a semiconductor base layer comprising a heavily dopedsemiconductor substrate of the first conductivity type and a lightlydoped semiconductor layer of the first conductivity type on thesubstrate, the substrate forming a bottom source electrode of the MOSFETdevice, the semiconductor body being formed on the semiconductor baselayer and the first surface of the semiconductor body being opposite thesemiconductor base layer.
 5. The method of claim 4, further comprising:forming a buried region of the second conductivity type on thesemiconductor base layer and under the drain region.
 6. The method ofclaim 1, further comprising: in each transistor cell, forming a firstcolumn of the first conductivity type under and in electrical contactwith the body contact region finger formed therein, the first columnextending through the lateral superjunction structure.
 7. The method ofclaim 6, further comprising: forming the thin semiconductor regions ofthe second conductivity type around the first column.
 8. The method ofclaim 6, further comprising: in each transistor cell, forming a secondcolumn of the second conductivity type under the conductive gate andspaced apart from the source region finger formed therein, the region ofthe semiconductor body under the conductive gate between the sourceregion and the second column forming a channel of the MOSFET device, thesecond column extending through the lateral superjunction structure andis electrically unbiased, wherein the second column distributes acurrent from the channel of the MOSFET device in an on-state to thedrain drift region formed by the lateral superjunction structure to becollected by the drain region, and the first column pinches off thesecond column in an off-state of the MOSFET device to isolate theconductive gate from a drain voltage at the drain region.
 9. The methodof claim 8, further comprising: forming the conductive gate, the sourceregion and the second column so that the current flowing under the gatein the channel of the MOSFET device flows parallel to the drain driftregion.
 10. The method of claim 6, wherein forming the first columncomprises: forming a plurality of blocking columns of the firstconductivity spaced apart along the body contact region and extend in adirection perpendicular to the drain drift region, each blocking columnextending through the lateral superjunction structure.
 11. The method ofclaim 1, wherein the thin semiconductor regions of the secondconductivity type in the lateral superjunction structure form draincurrent paths to carry the drain drift current in the on-state of theMOSFET device from the channel to the drain region; and the thinsemiconductor regions of the first conductivity type formcharge-balanced partition region to block the drain current paths in theoff-state of the MOSFET device.
 12. The method of claim 1, whereinforming the one or more lightly doped termination columns comprises:forming a one-dimensional array of termination columns extending in adirection away from the end of each heavily doped region finger.
 13. Themethod of claim 1, wherein forming the one or more lightly dopedtermination columns comprises: forming a two-dimensional array oftermination columns arranged at the end of each heavily doped regionfinger and having a pattern selected to optimize a breakdowncharacteristic of the MOSFET device.
 14. The method of claim 1, whereinforming the one or more lightly doped termination columns comprises:forming a two-dimensional array of termination columns arranged at theend of each heavily doped region finger and having a pattern selected tooptimize a breakdown characteristic of the MOSFET device.
 15. The methodof claim 1, wherein forming the termination structure comprises: forminga conductive field plate on the semiconductor body and surrounding atermination column.
 16. The method of claim 15, wherein forming theconductive field plate comprises: forming a multi-step field plateincluding a conductive field plate as a first step and a dielectricfield plate in subsequent steps.
 17. The method of claim 15, whereinforming the conductive field plate comprises: forming a polysiliconfield plate as the conductive field plate.
 18. The method of claim 1,wherein forming the termination structure comprises: forming a reducedsurface field shallow implant region having the same conductivity typeas the heavily doped region finger and positioned near the end of theheavily doped region finger, the reduced surface field shallow implantregion having a shape selected to optimize a field shaping effect forthe MOSFET device.
 19. The method of claim 18, wherein forming thereduced surface field shallow implant region comprises: forming thereduced surface field shallow implant region has a triangular shape or arectangular shape.
 20. The method of claim 1, wherein the firstconductivity type comprises P-type conductivity and the secondconductivity type comprises N-type conductivity.